Thesis (M.A.)--Boston University / In spite of the rapid development of the study of plant viruses and virus diseases there is still no generally accepted definition of a virus. Cook (1946) divided the history of plant virus study into three periods. The first dated from 1576, with the first published description of a virus disease, the breaking of tulips by Carolus Clusius and ended in 1868 with a description of the variegation of Abutilon striatum Dicks..The second began in 1882 with Mayer's work on tobacco mosaic. The third started in 1906 when the study of plant viruses was really beginning. Smith (1948) suggested adding a fourth period commencing in 1935 with Stanley's crystallization of the tobacco mosaic virus as a definite entity. Since then the physicist, the biochemist, and the serologist have joined in the research. Estimates of losses due to plant virus diseases are difficult to make because of varying conditions. Bawden (1950) stated that the one thing common to all plant viruses is that they are nucleoproteins containing a ribose nucleic acid. [TRUNCATED]

Cette thèse porte sur l'interaction entre turbulence et écoulement au bord du plasma de tokamak, et leur influence sur le confinement. La turbulence est la principale contribution au transport dans les machines de fusion magnétique, et un facteur limitant leur performance. Elle peut être stabilisée par les écoulements dans le plasma, via la décorrélation des cellules de convection par le cisaillement, et des couplages non-linéaires. La réduction localisée du transport turbulent par un écoulement cisaillé (barrière de transport) est souvent observée dans les expériences, et des régimes à confinement amélioré tels que le mode à Haut confinement sont obtenus sur de nombreuses machines. Les expériences tendent à montrer que l'écoulement moyen responsable de la barrière est gouverné par l'équilibre des forces, mais qu'il existe une dynamique complexe entre la turbulence, les écoulements zonaux et l'écoulement moyen pendant la phase de transition. Dans cette thèse, nous enrichissons un modèle fluide de turbulence du plasma de bord afin d'inclure la relaxation collisionelle de l'écoulement vers l'équilibre des forces. Nous montrons que la contribution des effets néoclassiques permet la formation spontanée d'une barrière de transport dans les simulations en forçage par un flux. Certains éléments dynamiques similaires à la transition L-H et au mode H sont recouvrés, tels que des relaxations de la barrière, ainsi que des oscillations du champ électrique lors de la formation de la barrière. Notre analyse montre que les écoulements zonaux causent une réduction temporaire de la turbulence via le couplage non-linéaire, ce qui permet l’établissement de la barrière. / The topic of this thesis is the interaction between turbulence and flows at the tokamak edge, and their influence on the confinement. Turbulence is the main contribution to the outward transport in magnetic fusion devices, and a strong limiting factor for their performance. It can be stabilised by flows, through shear-mediated decorrelation of convective cells, and through non-linear coupling. Strong shear flows causing a localised reduction of transport (transport barrier) are often observed in experiments, and several regimes of improved confinement such as the High-confinement mode are accessed routinely. There is a growing body of evidence from experiments showing that the mean flow responsible for the barrier is governed by force balance, while non-linear interplay between turbulence, turbulence-driven zonal-flows, and the mean flow occurs during the transition phase. In this thesis, we extend a fluid model for plasma edge turbulence to include collisional relaxation of flows towards force. We show that accounting for a contribution of neoclassical allows for the spontaneous formation of a transport barrier to occur in flux-driven simulation. Dynamical features reminiscent of the L-H transition and H-mode are recovered, such as relaxation-oscillations of the barrier and dithering of the radial electric field during the barrier formation. An analysis is carried out to identify the roles of zonal-flows and force balance during the transition, and it is found that in our simulations that zonal flows provide temporary quenching of the turbulence via non-linear coupling, allowing for the mean flow to grow and form the barrier.

Evolution of measured profiles of densities, temperatures and velocities in the edge pedestal region between successive ELM (edge-localized mode) events are analyzed and interpreted in terms of the constraints imposed by particle, momentum and energy balance in order to gain insights regarding the underlying evolution of transport processes in the edge pedestal between ELMs in a series of DIII-D discharges. The data from successive inter-ELM periods during an otherwise steady-state phase of the discharges were combined into a composite inter-ELM period for the purpose of increasing the number of data points in the analysis. These composite periods were partitioned into sequential intervals to examine inter-ELM transport evolution. The GTEDGE integrated modeling code was used to calculate and interpret plasma transport and properties during each interval using particle, momentum, and energy balance. Variation of diffusive and non-diffusive (pinch) particle, momentum, and energy transport over the inter-ELM period are examined for discharges with plasma currents from 0.5 to 1.5 MA and inter-ELM periods from 50 to 220 ms. Diffusive transport is dominant for ρ< 0.925, while non-diffusive and diffusive transport are very large and nearly balancing in the sharp gradient region 0.925 <ρ <1.0. Transport effects of ion orbit loss are significant for ρ > 0.95, and are taken into account. During the inter-ELM period, diffusive transport increases slightly more than non-diffusive transport, increasing total outward transport. Both diffusive and non-diffusive transport have a strong inverse correlation with plasma current. Weakening the electromagnetic pinch may increase outward particle transport, and enable control over the rebuilding of the edge pedestal between ELMs.

Edge localized modes (ELMs) are a concern for future magnetic fusion devices, such as ITER, due to the large transient heat loads they generate on the plasma facing components. A very promising method of ELM suppression is an application of resonant magnetic perturbations (RMP); however, such application leads to localized places of higher heat fluxes called footprints. Both ELMs and RMP could limit the operational lifetime of the device. In this thesis, we analyze the temporal and spatial distribution of footprints using the tangle distance method in the aim to prevent a transient overheating. We also analyze quasi-double-null configuration of the ITER plasma which can be expected to be the most susceptible to overheating of the upper wall. Based on the modelling, the potentially dangerous configurations of the RMP have been shown. Using the ELM filament model included in the LOCUST GPU code, we study temporal and spatial distribution of the heat fluxes caused by ELMs in the axially symmetric and the asymmetric magnetic field. The results are compared with published experimental observations. Powered by TCPDF (www.tcpdf.org)

Dans les tokamaks, les instabilités nommées ELMs (pour ``Edge Localized Modes'') génèrent des relaxations quasi-périodiques du plasma, potentiellement néfastes pour le divertor d'ITER. Une méthode de contrôle des ELMs prévue pour ITER est l'application de Perturbations Magnétiques Résonantes (RMPs), capables de mitiger ou supprimer les ELMs dans les tokamaks existants. Afin d'améliorer la compréhension de l'interaction entre les ELMs, les RMPs et les écoulements du plasma et de réaliser des prédictions fiables pour ITER, la simulation non-linéaire des ELMs et des RMPs est réalisée avec le code de MHD réduite JOREK, en géométrie réaliste. Les effets bi-fluides diamagnétiques, la friction poloidale néoclassique, une source de rotation parallèle et les RMPs ont été ajoutés dans JOREK pour simuler la pénétration des RMP en prenant en compte la réponse cohérente du plasma. Dans un premier temps, la réponse du plasma aux RMPs (sans ELMs) est étudiée dans le cas des tokamaks JET, MAST et ITER, pour des paramètres réalistes. Ensuite, la dynamique cyclique des ELMs (sans RMPs) est modélisée pour la première fois en géométrie réaliste. La compétition entre la stabilisation du plasma par la rotation diamagnétique et sa déstabilisation par la source de chaleur induit la reconstruction cyclique du piédestal. Enfin la mitigation et la suppression des ELMs sont obtenues pour la première fois dans nos simulations. Le couplage non-linéaire des RMPs avec des modes instables du plasma induit une activité MHD continue à la place des violentes relaxations d'ELMs. Au-delà d'un seuil de perturbation magnétique, la suppression totale des ELMs est également observée. / The growth of plasma instabilities called Edge Localized Modes (ELMs) in tokamaks results in the quasi-periodic relaxations of the edge plasma, potentially harmful for the divertor in ITER. One of the promising ELM control methods planned in ITER is the application of external resonant magnetic perturbations (RMPs), already efficient for ELM mitigation/suppression in current tokamak experiments. However a better understanding of the interaction between ELMs, RMPs and plasma flows is needed to make reliable predictions for ITER. In this perspective, non-linear modeling of ELMs and RMPs is done with the reduced MHD code JOREK, in realisitic geometry including the X-point and the Scrape-Off Layer. The two-fluid diamagnetic drifts, the neoclassical friction, a source of parallel rotation and RMPs have been implemented to simulate the RMP penetration consistently with the plasma response. As a first step, the plasma response to RMPs (without ELMs) is studied for JET, MAST and ITER realistic plasma parameters and geometry. Then the cyclic dynamics of the ELMs (without RMPs) is modeled for the first time in realistic geometry. After an ELM crash, the diamagnetic rotation is found to be instrumental to stabilize the plasma and to model the cyclic reconstruction and collapse of the plasma pressure profile. Last the ELM mitigation and suppression by RMPs is observed for the first time in modeling. The non-linear coupling of the RMPs with unstable modes is found to induce a continuous MHD activity in place of a large ELM crash, resulting in the mitigation of the ELMs. Over a threshold in magnetic perturbation, the full ELM suppression is also observed.

Next-generation and present fusion devices such as the Joint European Torus (JET) aim for plasma operations in H-mode, a plasma regime with high confinement and low loss of energy and fuel that results from a barrier for heat and particle transport at the plasma edge, the pedestal. Due to steep gradients of the density, temperature and pressure profiles, the pedestal experiences instabilities called Edge-localized-modes (ELMs) which lead to large fluxes of heat and particles that might damage machine components. A theoretical value for the critical threshold for the pressure gradient is determined by the Peeling-Ballooning (PB) model. Although this model has been rather reliable, experiments have shown a discrepancy between the experimental pedestal pressure gradient and the critical pressure gradient determined by the model. A number of experimental gradients were significantly lower than the predicted gradients. The mechanisms responsible for this discrepancy are not fully understood yet. The present hypothesis identifies the relative shift between the positions of the temperature and density pedestals and neutral pressure as key parameters, related to input power and gas dosing among other engineering parameters. Further impact could arise from the assumption of equal ion and electron temperature and the neglecting of the plasma rotation (velocity). In order to investigate this issue more thoroughly, JET established a comprehensive database containing pedestal characteristics. As a part of this work, a software has been implemented to visualize the data since such a tool did not exist yet. The tool enables the plotting of pedestal related parameters while specific data subsets can be selected or neglected. The tool has been used to investigate the impact of ion temperature and plasma rotation on the discrepancy between the theoretical and experimental critical gradients. Hereby, many relevant parameters needed to be constrained to observe an isolated impact of ion temperature and plasma rotation. The results of this investigation support the hypothesis that mainly the relative shift affects the discrepancy between experimental and predicted pressure gradient, but that also the use of experimental ion temperature can contribute to reduce the discrepancy. / Nästa generations och nuvarande fusionsanordningar, såsom Joint European Torus (JET), syftar till plasmaoperationer i H-läge, en plasmaregim med hög inneslutning och låg förlust av energi och bränsle som är ett resultat av en barriär för värme­ och partikeltransport vid plasmakanten, piedestalen. På grund av branta gradienter i densitets-, temperatur- och tryckprofilerna, är piedestalen instabil. Instabiliteten kallas Edge-Localised-Mode (ELM) och leder till stora flöden av värme och partiklar som kan skada maskinkomponenter. En kritisk tröskel för tryckgradienten bestäms av modellen Peeling-Ballooning (PB). Även om denna modell har varit ganska tillförlitlig, har experiment visat en diskrepans mellan den experimentella piedestaltryckgradienten och den kritiska tryckgradienten som bestäms av modellen. Ett antal experimentellt uppmätta kritiska gradienter är signifikant lägre än de förväntade enligt modellen. De mekanismer som ligger bakom denna diskrepans är ännu inte helt klarlagda. Den nuvarande hypotesen identifierar skillander i pos positionen av temperature- och täthetspiedestalen, samt trycket från neutraler som nyckelparametrar, relaterade till inmatad effekt och gasdosering bland andra ingenjörparametrar. Ytterligare påverkan kan uppstå genom antagandet att joner och elektroner har samma temperatur och försummandet av plasmarotationen. För att undersöka denna fråga mer ingående upprättade JET en omfattande databas med piedestalegenskaper. Som en del av detta arbete har en programvara implementerats för att visualisera data eftersom ett sådant verktyg inte funnits tidigare. Verktyget möjliggör plottning av piedestalrelaterade parametrar medan specifika data delmängder kan väljas eller väljas bort. Verktyget har använts för att undersöka inverkan av jontemperatur och plasmarotation på diskrepansen mellan teoretiska och experimentell kritiska tryckgradienten. För denna analys behövde många relevanta parametrar begränsas för att observera en isolerad påverkan av jontemperatur och plasmarotationen. Resultaten av denna undersökning stödjer hypotesen att det relativa skiftet främst påverkar diskrepansen mellan experimentell och förutsedd tryckgradient men att även användning av experimentell jontemperatur kan bidra till att minska diskrepansen.